Method of casting aluminum



Aug. 29, 1967 R. T. CRAIG 3,338,296

METHOD OF CASTING ALUMINUM Filed April 2, 1964 IN V EN TOR.

E/chard T Craig ATTORNEY United States Patent 3,338,296 METHOD OF CASTING ALUMINUM Richard T. Craig, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 2, 1964, Ser. No. 356,799 3 Claims. (Cl. 164-89) The invention relates to improvements in the continuous casting of tin-containing aluminum alloy ingot or shapes. The principal object of the invention is to provide a method of obtaining good ingots of said aluminum alloy, particularly when molds made of aluminum or aluminum =base alloy are employed to form the ingot during the casting process. The essentials of the so-called continuous casting process, are that the molten metal to be cast is continuously fed into one end of an open-ended mold, or tube, in which the metal, by reason of removal of heat of solidification, is at least partially solidified in ingot shape and from which it is withdrawn at a rate substantially equal to the rate of feed of said molten metal, whereby an ingot or shape of constant cross-section is continuously solidified until the desired length, or a multiple of said length, is obtained. Such processes are conventional to the commercial production of rolling, forging and extrusion ingot from aluminum base alloys. The molds used in such processes are shaped in crosssection to produce the cross-sectional area of ingot desired and are quite short in length being, usually, but -a few inches or a foot or so since the head of molten metal actually maintained in the mold during the casting process is not great. The molds are usually made of a high conductivity metal in respect of heat, such as aluminum or copper or alloys of either metal, either in wrought or other form and where made of aluminum or aluminum base alloy are herein, and in the appended claims, referred to as aluminum molds. During the casting operation coolant, such as water, is applied to the outer surface of the aluminum mold and, usually, to the solidified or partially solidified metal ingot issuing from said mold, all to remove heat of solidification and to effect the solidification of the metal intoingot at the cooling rate desired.

In these essentials such a continuous casting operation may be illustrated schematically by the figure of the attached drawing where, in partial cross-sectioned elevation, is indicated a conventional ingot forming operation wherein an open ended aluminum mold 10, associated with coolant providing pipes 11, is mounted in fiixed position over a platform 12 which is attached to and supported on a lowering device 14. The platform 12 initially forms the bottom of mold 10, but once the casting process is started it is lowered as the ingot is formed, allowing withdrawal of the ingot from the mold at the rate it is formed, or its shell is formed, therein. In the figure the casting is shown in progress, the molten metal 13 being fed to the operation through the spout 9 and at arate which, taking into account the coolant, the rate of solidification and the lowering rate of platform 12, maintains the desired level, or head, 15 in the aluminum mold 10.

Often used in, but not necessary to, such processes is a vibrator, indicated at 20, such as the type used in the sand molding art to release the pattern from the mold, i.e., a vibrator having a range up to about 3600 vibrations per minute. Usually desirable to the production of satisfactory castings is some form of lubrication of those surfaces of mold which contact the metal being solidified. In the drawing one means of providing such lubrication is shown at 16, a suitable oil being fed by such means through openings 17 to the inner surfaces of the mold 10 during the casting operation. As is well known, arrangements may be made to dispense with an intermediate pouring device, such as shown at 9, and the mold 3,338,296 Patented Aug. 29, 1967 may be directly attached to the wall of a furnace or other molten metal receptacle so that the molten metal to be cast is directly fed to the mold. Likewise, by suitable and known mechanical arrangements, the movement of the ingot during the casting operation may be horizontal or at an angle to the vertical. In any event, however, the essentials of the so-called continuous method, such as are described above and indicated in the drawing, remain and it is to this method that the present invention relates.

In the casting of tin-containing aluminum base alloy by such methods, it has not, heretofore, been possible to obtain satisfactory ingot, difiiculty being that the resultant ingot has torn or disrupted surfaces which, on later working, by rolling, forging or other method produce, or tend to produce, cracks or defects in the worked metal. During the casting process these tears or defects appearing on the surface of the'tin-containing aluminum ingot are quite severe and may result in actual rupture of the ingot surface to such a depth that, at times, the asyet-unsolidified metal in the central porton of the ingot breaks through the thus weakened solidified ingot shell with consequent disruption of the casting process. The tendency of tin-containing aluminum alloy to thus unfavorably react to these conventional casting processes is present even when the tin is present in the alloy in amounts as low as about 0.05% by weight and is likewise present when the tin is present in greater amounts, such as, for instance, 20 to 30% by weight of the alloy. The presence of other alloying elements in the aluminum do not appear to either diminish or enhance the difiiculty encountered with these tin-containing aluminum alloys and it would appear, for all practical purposes, that whenever tin is present in amounts essential to impart some useful property to aluminum base alloy, or even as an impurity in substantial amounts such as 0.03% by weight, the tendency to surface impairment of the ingot during continuous casting exists. Thus the surface tearing or disruption above mentioned has been encountered in the continuous casting of ingot from commercial tin ingot from commercial tin containing alloys varying as widely in composition as the following:

(a) 0.05% to 0.2% tin, 6 to 8% zinc, balance aluminum and impurities;

(b) 5.5% to 7% tin, l to 2% silicon, 0.1 to 1.3% copper, 0.2 to 0.7% nickel, balance aluminum and impurities;

(c) 20% tin, 1% copper, balance aluminum and impurities; and

(d) 6.5% tin, 2% copper, 1.2% nickel, 0.75% magnesium,ba1ance aluminum and impurities;

all percentages in the above alloys being by weight.

I have found that the above mentioned defects of surface tearing and disruption in tin-containing aluminum ingot during continuous casting may be substantially avoided if there be provided between the molten metal in the aluminum mold and the aluminum mold surface a thin layer of chromium such as is usually produced by electro-deposition. When such a chromium layer is provided the effect appears to be specific; i.e., the tearing and disruption of the surface of the forming ingot ceases and the ingot produced is, as to surface quality, comparable to those usually produced in aluminum molds by the continuous casting therein of alloys not containing tin. This chromium layer or coating does not lessen the desirability of the lubrication usually employed in the continuous casting of aluminum or its alloys into ingot nor does is change the conditions which, as is well known, sometimes normally recommend the use of vibration devices to vibrate the mold. It would appear, however, that the chromium surface or coating in some way prevents a specific surface wetting or attraction effect between the tin containing molten aluminum and the surface of the aluminurn molds usually employed in the continuous casting process and by such preventative action stops the tearing and disruption of the ingot surface. Whatever may be the effect, the principle of the invention may likewise be employed when molds made of metal other than aluminum, such as copper or bronze, employed as the mold material in the aforesaid casting processes and the tin-containing aluminum is subjected to said process.

In the commercial practice of the invention, I have not found that a special process is needed to provide a thin chromium layer between the molten tin-containing aluminum and the wall of the mold in which the continuous casting takes place. Since it is usually an object of a continuous casting process that there be as little interference as possible with the transfer of heat from the metal being solidified to the coolant, an objective which has made desirable the use of thin walled molds built of high heat conductive material, such as aluminum or copper, it is axiomatic that the chromium layer be as thin as possible consistent with effective covering of the mold and subsequent wear. Since the chromium layer has no structural function, thicknesses of 2.5 to 5 mils will usually be sufficient and such coatings in such thicknesses can be readily provided by standard chromium electro-plating processes. These processes may or may not involve application of a preliminary coating, all as well known in the plating art. While the inner surface of the casting mold should be smooth, since such smoothness is usually desirable in continuous casting of aluminum and its alloys, the applied chromium electro-coating, if electro-coating is used, need not be bright in finish but may be, if desired, dull. However, .a light abrasive polishing of the surface is useful in removing any adhering foreign particles. The chromium layer or surface need be interposed only between the molten metal and the mold surface, conseqeuntly a coating the full length of the inner surface of the mold is not necessary where the level or head 15, as shown in the drawing, is not in contact with the upper portion of the mold. Where, however, direct liquid head casting from the floor of a furnace, or other molten metal holding receptacle, is employed the entire inner surface of the mold should be protected by the chromium layer.

Using a continuous casting process of the type schematically indicated in the drawing, the method of the present invention has been used to cast, without serious surface fdifliculties, commercial amounts of ingots for rolling from a tin-containing aluminum alloy of the following composition; about 6% tin, 1.5% silicon, about 0.5% copper and 0.5% nickel. This alloy has previously been found to be subject to severe surface tearing and disruption when cast by such a process in a noncoated aluminum mold for this reason, commercial casting of the ingot and frequency used vibration as a necessary, as distinguished from a desirable, adjacent. Even when vibration has been used, surface tearing and disruption has been present in highly undesirable amounts. By interposing a layer of chromium between the mold and the molten metal present therein, in accordance with this invention, it Was found possible to continuously cast this alloy into rolling ingot without substantial surface tearing or disruption, whether or not vibration was used. In one commercial run, using 5 /2 inch deep aluminum molds having a cross-section of 8 inches by 30 inches and provided on their inner mold surface with an electro-deposited layer of chromium of 2.5 mils in thickness, over 300,- 000 lbs. of ingots, each 63 inches in length, were cast from an alloy of the above mentioned type at a rate of 3 inches per minute of ingot formation. No surface disruption or tearing of the type previously encountered in the use of this alloy was observed.

Having thus described my invention, I claim:

1. In a method of casting molten tin-containing aluminum base alloy into ingot by providing an open-ended mold of metal of relatively high heat coductivity, continuously feeding said molten alloy into said mold and withdrawing heat of solidification from said alloy to effect in said mold at least partial solidification of ingot in said mold, and continuously withdrawing said thus formed ingot from said mold, the improvement of controlling surface disruption and tearing of said ingot by interposing between said molten alloy and the inner metal surface of said mold throughout said casting process in a layer of chromium, said layer being attached to said surface.

2. Process of claim 1 characterized by the fact that an aluminum mold is employed.

3. Process of claim 1 characterized by the fact that said layer of chromium is electro-deposited on said inner metal surface.

References Cited UNITED STATES PATENTS 2,759,231 8/1956 Parlanti 249-114 X 2,763,040 9/ 1956 Korb 22200.1 X 2,837,791 6/1958 Tessmann 22-57.2 2,983,972 4/1961 Moritz 22--57.2 3,186,678 6/1965 Keating 22192 1. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner. 

1. IN A METHOD OF CASTING MOLTEN TIN-CONTAINING ALUMINUM BASE ALLOY INTO INGOT BY PROVIDING AN OPEN-ENDED MOLD OF METAL OF RELATIVELY HIGH HEAT CONDUCTIVITY, CONTINUOUSLY FEEDING SAID MOLTEN ALLOY INTO SAID MOLD AND WITHDRAWING HEAT OF SOLIDIFICATION FROM SAID ALLOY TO EFFECT IN SAID MOLD AT LEAST PARTIAL SOLIDIFICATION OF INGO IN SAID MOLD, AND CONTINUOUSLY WITHDRAWING SAID THUS FORMED INGOT FROM SAID MOLD, THE IMPROVEMENT OF CONTROLLING SURFACE DISRUPTION AND TEARING OF SAID INGOT BY IN TERPOSING BETWEEN SAID MOLTEN ALLOY AND THE INNER META SURFACE OF SAID MOLD THROUGHOUT SAID CASTING PROCESS IN A LAYER OF CHROMIUM, SAID LAYER BEING ATTCHED TO SAID SURFACE. 